A conventional steering system for a vehicle generally comprises a handling mechanism consisting of a steering wheel; a steering shaft and a column; a gear mechanism consisting of a steering gear mounted on a frame and a link; and a link mechanism consisting of a pitman arm, a drag link and a tie rod.
Such steering systems are designed to adjust the directional orientation of front wheels by gearing the tie rod to the left or right in response to the rotating direction of the gear installed at the end of the steering shaft.
However, since the steering systems are designed so that only the front wheels are steered, the vehicle body is inclined suddenly and unstable during high speed turns.
Accordingly, 4-wheel steering systems which can steer the front wheels and the rear wheels simultaneously have been recently developed.
As shown in FIG. 1, the front and rear wheels generally maintain a counter-phase angle below a predetermined velocity range of about 40-60 km/h to improve handling ability at lower speeds while maintaining the same-phase above the predetermined velocity range to improve driving ability at higher speeds.
In earlier developments of the 4-wheel steering system, the system was designed such that the steering force was transmitted by a hydraulic system. In recent years, however, the steering force is transmitted with an actuating motor to simplify the structure and improve controllability.
FIG. 2 shows a conventional 4-wheel steering system using the actuating motor. The motor 1 is controlled by an electronic control unit ("ECU") to which information relating to front wheel angle and vehicle speed is inputted at all times. A timing belt 2 is connected to a shaft 11 of the motor 1. The timing belt 2 is further connected to a pulley (not shown). The pulley is provided with a screw groove on its inner circumference. A ball screw 3 is screwed into the screw groove such that the rotary motion of the motor 1 can be converted into a linear motion of the ball screw 3. One end of the ball screw 3 is pivotally coupled with one end of a link 4. The other end of the link 4 is fixed to one end of a tie-rod 5, the other end of which is fixed to a rear wheel 6. In addition, a compression spring 7 is mounted on the other side of the ball screw 3 to provide dynamic stability to the ball screw 3 such that the spring can align the rear wheel 6 in the same direction as that of the longitudinal direction of the vehicle when the power of the motor 1 cannot be transmitted to the rear wheel 6.
The operation of the above-described 4-wheel steering system will be described hereinafter with reference to FIG. 3.
When the front wheel angle is determined by manipulation of the steering wheel, both the front wheel angle and the current vehicle speed are inputted into an operation part 20 of the ECU. A desired rear wheel angle is then determined by the operation part 20. This desired rear wheel angle is transmitted to a controller 21 which generates a control signal that is amplified by an amplifier 22, to drive the motor 1.
Accordingly, the shaft 11 of the motor 1 rotates in a predetermined number of rotations. This rotating force is transmitted to the ball screw 3 through the timing belt 2, thereby linearly moving the ball screw 3. Therefore, the tie rod is pushed or pulled by the link 4 which is linearly moved in response to the linear movement of the ball screw 3 to steer the rear wheel.
At this point, when the practical steered angle of the rear wheel is measured, this steered angle is compared with the desired rear wheel angle. If there is difference between the angles, the differential value is compensated by the controller 21 such that the practical steered angle becomes the same as the desired rear wheel angle.
Accordingly, to achieve the above described feedback, the practical (or actual) rear wheel angle must be measured. A well-known method for measuring the rear wheel angle is a combination method utilizing an encoder method and a potentiometer method.
According to the encoder method, the rear wheel angle or rotating angle of the motor 1 is measure by a pulse signal. Although this method can obtain an exact measured value, there are shortcomings in that both a special compensating algorithm for converting the pulse signal into an angle, and an algorithm for converting a relative rear wheel angle into an absolute rear wheel angle, are required. According to the potentiometer method, a resistance value is measured in accordance with a voltage value, and an absolute value of the rear wheel angle is directly measured from the resistance value. Therefore, since a special compensating algorithm is not required, the measurement of the rear wheel angle can be quickly achieved. The degree of precision, however, is reduced due to current noise when applying voltage.
Accordingly, as described above, a combination method of the encoder and the potentiometer method has been used so that mutual compensation can be achieved. Namely, the potentiometer is used for measuring the rear wheel angle in short time, and the encoder method is used for compensating the error of the potentiometer method and controlling an initial setting.
In the above-described rear wheel controlling method, the rear wheel angle is controlled by a point-to-point method on the basis of a frame time.
To further explain, when both a rear wheel angle and a vehicle speed are inputted into the ECU thereby calculate a desired rear wheel angle, a feedback control is performed by a controller to compensate for an error value. The error value is the difference between the desired rear wheel angle and the practical rear wheel angle. At this point, the point-to-point method is used, in which the frame time which is a control cycle is set at about 10 m/s. The rear wheel is controlled by a constant desired rear wheel angle during a special single cycle. After this special cycle, the rear wheel is controlled by a newly-calculated desired rear wheel angle.
However, in a rear wheel control device of the 4-wheel steering system, when the device malfunctions, a gain value of the controller should be increased to allow the controller to have strength against external conditions such as a direction change of a compression spring for returning the rear wheel to a neutral position and an external force change caused by the rear wheel. To enable the controller to read the error value and control it, a certain value should be multiplied to an error value. The certain value is the gain value.
The direction of the compression spring will be described more in detail hereinafter. When the rear wheel is steered from a neutral position, the biasing force of the spring acts in a direction restraining the steering operation. However, when the rear wheel is returned to the neutral position, the biasing force of the spring acts in a direction urging the return of the rear wheel.
However, in the conventional point-to-point method, since the gain value is constant regardless of a direction of the pre-pressure spring, a response value of the actuating motor, with respect to a signal generated from the controller, becomes too small when the pre-pressure spring acts in a direction restraining the steering operation. When the pre-pressure spring acts in a direction urging the return of the rear wheel, the response value becomes too large.
When increasing the gain value, the problems of the former can be solved. However, in case of the latter, since the response value of the actuator is further increased, an over-shoot phenomenon occurs. On the other hand, when decreasing the gain value, responsiveness is deteriorated.